1. Field of the Invention
The invention relates to a brake system for a motor vehicle including an actuating device constructed as an electro-mechanical wheel brake actuator mounted at a brake caliper of a wheel, the wheel brake actuator having a spindle driven in axial direction by an electric motor.
The presently ever increasing demands made of a modern brake system for motor vehicles, such as anti-lock systems, stability controls, drive slip or traction controls, necessitate wheel-selective brake interventions. So far, that has been carried out with conventional brake systems expanded with hydraulic pumps and magnetic valves, as in German Patent DE 29 54 162 C2. However, vibration problems in the hydraulic lines then arise along with difficult triggering of pressure modulation units, that is magnet valves. Due to the properties of the magnet valves, which are high-quality non-linear two-point links, the control quality with respect to the brake pressure is also limited. Such brake systems also involve considerable effort for installation in the motor vehicle: brake lines have to be laid and connected, the brake system has to be filled with brake fluid and bled, and the intactness of the system must be checked. Even during operation, there is not-inconsiderable maintenance expense, especially for the regular replenishment of brake fluid and for disposing of used brake fluid in an environmentally appropriate way.
At present, all of the known manufacturers are still equipping their vehicles with conventional brake systems. Wheel-selective brake interventions are accomplished through the use of hydraulic pumps and magnetic valves, but at the cost of the disadvantages discussed above. In order to provide a gentle buildup of brake pressure, for instance in the case of cruise control and distance-keeping devices, some manufacturers use electronically regulated vacuum brake boosters. In order to suppress the problems of vibration and the attendant noise production, proportional valves and pressure reservoirs can be used. However, the proportional valves in particular increase the price of the brake system. Additionally, they do not eliminate the disadvantages associated with the hydraulic fluid.
A brake system of that generic type, such as in German Published, Non-Prosecuted Patent Application DE 195 11 287 A1, has an electromagnetically actuated disk brake with a floating caliper and an actuation unit (or brake actuator) secured to the caliper. The brake actuator includes an electric motor, which acts through a step-down gear to axially displace a threaded spindle and thus press brake linings in pairs against a brake disk. The step-down gear is constructed as a roller thread drive in the form of a planetary gear. The rotor of the motor is secured to the threaded nut that forms the ring gear of the planetary gear. The planet wheels are constructed as elongated threaded rollers. That kind of step-down gear is complicated and expensive to produce.